The present invention relates to a device for curing a UV coating, in particular a UV paint coating, or of UV printing dyes, on a substrate, in particular on heat-sensitive materials.
Cold light UV irradiation devices are used in the coating of substrates of heat-sensitive materials, particularly synthetics, with UV paints and printing dyes. The substrates may be present in the shape of formed objects (bottles, discs, etc.) or as foils and strips. Disc-shaped objects may be optical information carriers such as Compact Discs (CD""s) or Digital Versatile Discs (DVD""s), for example. Other temperature-sensitive irradiation goods are ceramic-type materials such as those used in electronic components, for example. Metal and synthetic parts used in electronic components are often temperature-sensitive as well.
A high UV light intensity is necessary to cure the UV paint and printing dyes within the short cycle times of high-volume production lines. Usually, UV light in the wavelength range of 200 to 400 nm is used for curing. However, all common light sources also emit the long-wave heat radiation (infrared radiation/IR radiation) in addition to the UV light required for curing. However, the long-wave heat radiation leads to deformation and brittleness of the substrate and is, therefore, undesirable.
It is known from DE 39 02 643 C2 to position the light source directly above the irradiation goods and to place two cold light mirrors behind the light source to reduce the heat radiation. The disadvantage is that a great heat portion reaches the substrate from the lamp because of the direct beam path.
G 901 46 52.2 and DE 440 942 6 show arrangements that lower the heat load of the object with a heat filter in the direct beam path. These heat filters consist of a coated quartz glass disc and only slightly reduce the infrared radiation to the substrate. Furthermore, the quartz glass discs also absorb a portion of the UV radiation.
From U.S. Pat. No. 4,048,490, an arrangement is known where the direct beam path to the substrate is prohibited. The direct beam path is guided by a reflecting barrier past the lamp to reflectors located underneath the lamp and from there to the substrate. The extremely long beam paths are a disadvantage of this arrangement. The UV intensity decreases with an increasing length of the beam path. Another disadvantage is that the barrier also reflects the heat radiation entirely, resulting in an insufficient separation of the UV and the IR radiation. Furthermore, this arrangement can illuminate the substrate only two-dimensionally because the lamp and barrier constitute two radiation sources. The complex geometric arrangement of the reflectors and the required distance between the barrier and lamp require a very large assembly space for such arrangements. Thus, they cannot be used in small production lines.
Known from DE 38 01 283 C1 is a device for curing a UV protective paint coating on flat objects, where a flat output nozzle is located between the device and the object and where inert gas, for example nitrogen, is provided via a feed line to said nozzle, which replaces the oxygen of the air during the irradiation process and can lead to better quality of the cured protective paint coating.
An UV lamp arrangement for curing photo-polymerizable materials is known from DE 26 22 993 A1. To remove the heat radiation that cannot be used for curing, the lamp is surrounded by a water jacket made of clear molten quartz. One embodiment has a semi-circular reflective coating directly on the quartz sheathing of the lamp. It focuses the radiation of the lamp generally in the direction of the focussing plane in the neighborhood of the substrate.
Based on this state-of-the-art, it is the objective of the invention to create a device for curing a UV coating that enables an effective separation of the UV radiation from the IR radiation in order to reduce the heat load of the substrate and at the same time to achieve a high UV intensity through short beam paths.
In one embodiment of the invention, it is possible to focus the UV radiation on the substrate.
According to a first exemplary embodiment of the present invention, this objective is achieved by a device for curing a UV coating, in particular a UV paint coating, or of UV printing dyes, on a substrate, in particular on heat-sensitive materials, with at least one light source that is located above the substrate, where the light of said light source can be directed to the UV coating via a reflector system for purposes of curing, where at least one barrier prevents, at least partially, the direct beam path of the light source from striking the substrate, characterized in that the UV radiation emitted by the light source is reflected by a UV reflection coating of the barrier through the light source to the reflectors located behind the light source, and the barrier includes at least one heat absorbing body that absorbs, at least partially, the heat radiation emitted by the light source.
According to a second exemplary embodiment of the present invention, this objective is achieved by a device for curing a UV coating, in particular a UV paint coating, or of UV printing dyes, on a substrate, in particular on heat-sensitive materials, with at least one light source that is located above the substrate, where the light of said light source can be directed to the UV coating via a reflector system for purposes of curing, where at least one barrier prevents, at least partially, the direct beam path of the light source from striking the substrate, characterized in that the UV radiation emitted by the light source is reflected by a UV reflection coating, which is directly applied to the light source, through the light source to the reflectors located behind the light source, and the barrier includes at least one heat absorbing body that absorbs, at least partially, the heat radiation emitted by the light source.
The device subject to the invention causes an effective separation of the UV radiation from the IR radiation by making it possible to absorb more than 90% of the IR radiation. Due to the minimized path length of the radiation, the UV intensity is comparable with that of conventional devices, such as the ones according to DE 39 02 643 C2, where the light source is located directly above the irradiation goods. Furthermore, the separation of the UV and the IR radiation allows for the employment of light sources with up to eight times the energy when compared to the light sources used thus far, without increasing the heat load of the substrate. In this manner, it is possible to achieve extremely short cycle times or high throughput speeds in the production lines.
By a special geometry of the barrier with shaping for the UV reflection coating and its location directly under the light source, the reflection of the UV radiation is realized through the light source instead of directing the radiation past the lamp as was common thus far. The UV reflection coating with a semi-circular cross-section located in the shape design partially surrounds the light source at its bottom side. At least 50% of the UV radiation that strikes the UV reflection coating is reflected through the light source onto the reflectors located behind the light source due to the shape and arrangement subject to the invention.
If the UV reflection coating is applied directly at the outer side of the light source according to the aforementioned second exemplary embodiment, the UV radiation is almost entirely reflected through the light source. The losses when the UV radiation passes through the glass body of the light source and the gas are relatively low. The path of the UV radiation is minimal. Since this solution does not require special shape designs for the reflection coating at the barrier in order to reflect the UV radiation through the light source, the barrier can be designed as a geometrically simple heat absorbing body, for example, as a plate.
The heat absorbing body of the barrier together with the UV reflection coating avoids the direct heat radiation path onto the substrate.
If UV paint is used where low-molecular components evaporate, the emission of these components is reduced because of the low heat development on the substrate.
An effective separation of the UV and the IR radiation is possible if the UV reflection coating at the barrier is part of a cold light mirror. The reflectors behind the light source, which are preferably designed as cold light mirrors as well, divert only the UV radiation that is required for curing at least in part past the barrier to the substrate.
In an advantageous embodiment of the invention, boreholes are provided in the barrier, through which cooling media and/or gases can be transferred. Cooling prevents the barrier from emitting or reflecting heat radiation. The absorbed heat radiation can be transferred to the cooling medium, but also to a cooling air stream if the heat absorbing body of the barrier is equipped with cooling fins that transfer the heat to a cooling air stream. Through cooling, the heat-absorbing body of the barrier can be kept at a constant temperature by regulating the amount of heat removed.
Using the boreholes, gases such as nitrogen can be transferred as well in order to sweep the substrate. In this manner, short curing times with optimal curing can be achieved. It is particularly advantageous to deploy the gas through wide boreholes in the shape of nozzles in the barrier directly above the substrate. However, gases cannot only be deployed using these additional boreholes but alternatively also suctioned off, for example, in order to prevent low-molecular materials emitted by coatings of lower quality to deposit on the reflectors.
To focus the UV radiation in one point, the reflectors that are positioned behind the light source are, at least partially, designed cylindrically with a semi-circular cross-section. The semi-circular cross-section of the reflectors focuses the radiation in one focal point on the substrate. However, if the aim is to achieve a two-dimensional illumination, it is useful to design the reflectors behind the light source, at least partially, in plate-shape.
Providing an asymmetric arrangement of the barrier and of the reflectors, behind the light source and disposed asymmetric to a vertical plane containing the longitudinal axis of the light source and being positioned perpendicular to the surface of the substrate, has the effect that the substrate initially pre-cures when running under the device and then is irradiated with high UV intensity. Such pre-curing results in a matte finish of the UV paint coating.
The intensity of the UV radiation can be varied by making the distance between the barrier and the light source adjustable, whereby the intensity decreases as the distance increases.
A small portion of heat radiation may be required to achieve optimal curing. The portion of the radiation that gets past the barrier system can be adjusted by using an aperture system to create an adjustable barrier geometry wherein the barrier includes an aperture system with height-adjustable apertures that allows for an adjustment of the radiation that will strike the UV coating of the substrate coming from the light source without being reflected. Heat apertures that can slide fully to the barrier and are located above the substrate, wherein the barriers are capable of fully shielding the substrate from the radiation of the light source also enable an adjustment of the radiation that strikes the substrate. They can also fully prevent radiation (shutter) and thus protect the substrate from too much UV radiation when the production line is at a standstill.
Adjustment capabilities of the apertures of the aperture system, for example, may be adjustable asymmetric to a vertical plane containing the longitudinal axis of the light source and being positioned perpendicular to the surface of the substrate and/or may be adjustable from the outside during the operation of the device. Such adjustments allow for an adaptation of the heat radiation affecting the substrate to changing production conditions (environmental temperature, air humidity, process speed, etc.) while the production is running.
The adjustment system may include, for example, a electrical or pneumatic drive.
A deflection of the lamp body is prevented because of the existence of at least partial contact between the light source and the barrier, especially through support structures. This allows for the employment of lamp bodies with lengths of up to 4 m, such as those that are necessary for paint curing on very wide packaging foils or of floor coverings, for example.